Plaque Characterization Using Multiple Intravascular Ultrasound Datasets Having Distinct Filter Bands

ABSTRACT

A system and method are disclosed that facilitate generating visual representations of characterized tissue based upon ultrasound echo information obtained from a portion of an imaged body. The system includes a first filter having a first filter band that is applied to a near range portion of the ultrasound echo information to render near range filtered echo information. A second filter, having a second filter band that covers a frequency range of the first filter band, is applied to a far range portion of the ultrasound echo information to render far range filtered echo information. The system furthermore includes a set of characterization criteria that are applied to the near and far range filtered echo information. The characterized near and far range image data are thereafter combined into a single tissue-characterization image.

FIELD OF THE INVENTION

The present invention generally relates to the field of imaging systems,and more particularly to intravascular imaging systems used to diagnoseand treat vascular disease.

BACKGROUND OF THE INVENTION

The development of new medical technologies has provided an increasingnumber of options available to doctors for the diagnosis and treatmentof cardiovascular diseases. The availability of such equipment hasimproved the ability of doctors and surgeons to detect and treatcardiovascular disease. Intravascular imaging technologies have enableddoctors to create and view a variety of images generated by a sensorinserted within a vasculature. Such images compliment traditionalradiological imaging techniques such as angiography by providing imagesof the tissue within vessel walls rather than showing a two dimensionallumen image.

Intravascular ultrasound (IVUS) analysis finds particular application toa system and method for quantitative component identification within avascular object including characterization of tissue. It should beappreciated that while the exemplary embodiment is described in terms ofan ultrasonic device, or more particularly the use of IVUS data (or atransformation thereof) to characterize a vascular object, the presentinvention is not so limited. Thus, for example, using backscattered data(or a transformation thereof) based on ultrasound waves or evenelectromagnetic radiation (e.g., light waves in non-visible ranges) tocharacterize any tissue type or composition is within the spirit andscope of the present invention.

Imaging portions of a patient's body provides a useful tool in variousareas of medical practice for determining the best type and course oftreatment. Imaging of the coronary vessels of a patient by techniquesinvolving insertion of a catheter-mounted probe (e.g., an ultrasoundtransducer array) can provide physicians with valuable information. Forexample, the image data indicates the extent of a stenosis in a patient,reveals progression of disease, helps determine whether procedures suchas angioplasty or atherectomy are indicated or whether more invasiveprocedures are warranted.

In an ultrasound imaging system, an ultrasonic transducer probe isattached to a distal end of a catheter that is carefully maneuveredthrough a patient's body to a point of interest such as within acoronary artery. The transducer probe in known systems comprises asingle piezoelectric crystal element that is mechanically scanned orrotated back and forth to cover a sector over a selected angular range.Acoustic signals are transmitted and echoes (or backscatter) from theseacoustic signals are received. The backscatter data is used to identifythe type or density of a scanned tissue. As the probe is swept throughthe sector, many acoustic lines are processed building up asector-shaped image of the patient. After the data is collected, animage of the blood vessel (i.e., an IVUS image) is reconstructed usingwell-known techniques. This image is then visually analyzed by acardiologist to assess the vessel components and plaque content. Otherknown systems acquire ultrasound echo data using a probe comprising anarray of transducer elements.

In a particular application of IVUS imaging, ultrasound data is used tocharacterize tissue within a vasculature and produce images graphicallydepicting the content of the tissue making up imaged portions of avessel. Examples of such imaging techniques for performing spectralanalysis on ultrasound echoes to render a color-coded tissue map arepresented in Nair et al. U.S. Pat. No. 7,074,188 entitled “System andMethod of Characterizing Vascular Tissue” and Vince et al. U.S. Pat. No.6,200,268 entitled “Vascular Plaque Characterization”, the contents ofwhich are incorporated herein by reference in their entirety, includingany references contained therein. Such systems analyze responsecharacteristics of ultrasound backscatter (reflected sound wave) data toidentify a variety of tissue types found in partially occluded vesselsincluding: fibrous tissue (FT), fibro-fatty (FF), necrotic core (NC),and dense calcium (DC).

When characterizing the response of tissue when exposed to ultrasoundwaves, parameter values are considered at a data point in an imagedfield. Based upon response characteristics of known tissue types, tissueat the data point is assigned to a particular tissue type (e.g. necroticcore). In a known system, a data set is acquired using a single filterhaving a relatively wide band (e.g., 10-30 MHz for a 20 MHz IVUStransducer). The filtered raw digital ultrasound data is processed, andrendered tissue response data is applied to a tissue characterizationcriterion to render a tissue type for particular locations within animaged blood vessel.

SUMMARY OF THE INVENTION

In accordance with the present invention a system and method areprovided for characterizing plaque wherein distinct filter bands areapplied to at least two sections (ranges) of echo signals received by anintravascular ultrasound probe. Furthermore, echo data obtained from afirst/near range filter is processed according to a first tissuecharacterization criterion and echo data obtained from a second/farrange filter is processed according to a second tissue characterizationcriterion. The distinct first and second filters and associatedcharacterization criteria facilitate retaining as much information aspossible about imaged tissue in the far range while limiting thepresence of false image artifacts (e.g., NC speckle) near thelumen-tissue border. Such system and method is applicable, for example,to systems that perform spectral analysis on ultrasound echoes to rendera color-coded tissue map.

BRIEF DESCRIPTION OF THE DRAWINGS

While the claims set forth the features of the present invention withparticularity, the invention, together with its objects and advantages,may be best understood from the following detailed description taken inconjunction with the accompanying drawing of which:

FIG. 1 illustrates a tissue-characterization system suitable forcarrying out the disclosed tissue/plaque characterization schemeincluding multiple characterization criteria applied to multiple rangesof tissue/plaque depth associated with IVUS echo information;

FIG. 2 is a table identifying related characteristics for a frequencysignature-based tissue characterization scheme including first andsecond characterization criteria associated with near range and farrange signal filters, respectively; and

FIG. 3 is a flowchart summarizing an exemplary set of steps for applyingfirst and second characterization criteria to near and far range echoinformation in accordance with an illustrative multiple characterizationcriteria scheme.

DETAILED DESCRIPTION OF THE DRAWINGS

The disclosed system and method for characterizing plaque in bloodvessels utilize at least two distinct filter bands to process ultrasoundechoes received at different ranges of radial distance from thetransducer probe (based upon the location of the lumen-tissue border).In an exemplary embodiment, a first filter, having a relatively narrowbandwidth, is applied to “near range” ultrasound echoes that arise frombackscatter of ultrasound within a vessel lumen and near thelumen-tissue border. A second filter, having a relatively broadbandwidth that completely covers the band of the first filter, isapplied to “far range” ultrasound echoes that arise from backscatter ofultrasound by tissue that extends beyond the “near range”. In general,the first/near range filter renders echo information that has less noisecontent than echo information rendered by the second/far range filter.The filter bands are applied to near and far range echo information in avariety of ways including: analog circuitry and digital circuitry aswell as hardware/firmware/software and combinations thereof.

Furthermore, first and second tissue classification criteria are appliedto the filtered echo information rendered by the distinct first andsecond filters, respectively. A first tissue classification criterion,formulated according to samples observed using the first/near rangefilter, is applied to the filtered echo data rendered by the relativelynarrower first/near range filter to characterize near range plaque. Thesecond tissue classification criterion, formulated according to samplesobserved using the second/far range filter, is applied to the filteredecho information rendered by the second/far range filter.

An exemplary IVUS (intravascular ultrasound) system includes anultrasonic probe device mounted upon a flexible elongate member forinsertion into vasculature. The system furthermore includes a computingdevice comprising memory for storing computer executable instructionsassociated with a plaque characterization application program anddigitized ultrasound backscatter/echo data for rendering imagesgraphically depicting characterized tissue/plaque in vesselcross-sections. In the detailed description of the exemplary embodimentthat follows, like element numerals are used to describe like elementsillustrated in one or more figures.

Turning initially to FIG. 1, a tissue/plaque characterization system 100is schematically depicted. An intravascular ultrasound console 110 iscommunicatively coupled to an IVUS catheter 120. The IVUS catheter 120comprises a distally mounted ultrasound transducer probe 122 thatacquires backscatter data (e.g., IVUS data) from a blood vessel. Inaccordance with known IVUS catheters, the catheter 120 is maneuveredthrough a patient's body (e.g., via a femoral artery) to a point ofinterest. The transducer probe 122 is then controlled, via the console110 to emit ultrasound pulses and thereafter receive echoes orbackscattered signals reflected from vascular tissue/plaque and blood.Because different types and densities of tissue absorb and reflect theultrasound pulse differently, the reflected data (i.e., IVUS data)signals transmitted back to the console 110 by the IVUS catheter 120, isconverted by characterization software into images of vascular objects.It should be appreciated that the IVUS console 110 depicted herein isnot limited to any particular type of IVUS console, and includes allultrasonic devices known to those skilled in the art (e.g., In-VisionGold and s5™ systems of Volcano Corporation). It should further beappreciated that the IVUS catheter 120 depicted herein is not limited toany particular type of catheter, and includes all ultrasonic cathetersknown to those skilled in the art. Thus, for example, a catheter havinga single transducer (e.g., adapted for rotation) or an array oftransducers (e.g., circumferentially positioned around the catheter) iswithin the spirit and scope of the present invention.

Known imaging applications executed on an IVUS console (e.g. console110) or a communicatively coupled computing device (e.g., computingdevice 130), render a variety of image types from received echoinformation. A first type of imaging application converts ultrasoundecho signal data into gray scale images reflecting the relative strengthof the echo signal returned by the objects within the transducer probe120's field of view. In such imaging applications, the relatively lightand dark regions indicate different tissue types and/or densities.

Other imaging applications, such as a tissue/plaque characterizationapplication 132 executed on the computing device 130 communicativelycoupled to console 110, renders tissue/plaque type information basedupon the spectral (i.e., frequency and power) characteristics of theecho information received by the console 110 from the catheter 120. Thefrequency information extracted from the echo information rendered bythe catheter 120 is compared to the frequency response signaturesassociated with particular types of tissue/plaque to render atissue/plaque characterization image. A data storage 134 stores thetissue/plaque characterization images rendered by the characterizationapplication 132 from the echo information received from the console 110.The data storage 134 is, by way of example, any of a variety of datastorage devices, including RAM, cache memory, flash memory, magneticdisks, optical disks, removable disks, SCSI disks, IDE hard drives, tapedrives, optically encoded information discs (e.g., DVD) and all othertypes of data storage devices (and combinations thereof, such as RAIDdevices) generally known to those skilled in the art.

In the illustrative example, the tissue/plaque characterizationapplication 132 exists as a single application comprising one or morecomponents. However, in other embodiments, the characterizationapplication comprises multiple applications executed on one or morecomputing devices (including multiple processor systems as well asgroups of networked computers). Thus, the number and location of thecomponents depicted in FIG. 1 are not intended to limit the presentinvention, and are merely provided to illustrate the environment inwhich an exemplary system operates. Thus, for example, a computingdevice having a plurality of data storage devices and/or a remotelylocated characterization application (either in part or in whole) iswithin the spirit and scope of the present invention.

In accordance with an illustrative embodiment, the characterizationapplication 132 utilizes two distinct near/far range filters andtissue/plaque characterization criteria when generating an imageidentifying types of tissue/plaque present in a vessel cross-sectionbased upon frequency analysis of ultrasound echo information. Turning toFIG. 2, an exemplary two-filter/two-classification criteriatissue/plaque characterization scheme is summarized for a 20 MHztransducer (i.e., a transducer that emits ultrasound at a centerfrequency of 20 MHz). As depicted, by way of example, in the tablepresented in FIG. 2, a first characterization criterion (Scheme 1) isapplied to echo information associated with a near range that radiallyextends 240 microns into tissue/plaque from the lumen-tissue border of avessel. The echo information in the near range passes through a firstfilter having a relatively narrow band (e.g., about 12-26 MHz ornarrower) prior to application of the first characterization criterion.The second characterization criterion (Scheme 2) is applied to echoinformation associated with a far range that begins at a depth/distancewhere the near range ends at the 240 micron depth in the tissue/plaquewhere the near range ends. The echo information in the far range passesthrough a second filter having a relatively broad band (e.g., about10-30 MHz). The second filter's band completely covers the frequencyrange of the first filter's band.

It is explicitly noted that the above example is provided for anexemplary 20 MHz stationary 64-element array transducer assembly.However, other embodiments include a different transducer (e.g., a 45MHz single rotating element IVUS probe), a different set of criteria,and associated border depth and filter bands. Furthermore, each of theexemplary filter bands will differ in accordance with alternativeexemplary embodiments.

As those skilled in the art will readily appreciate, thecharacterization criteria will differ in accordance with differences ina variety of factors. Examples of such factors include, for example: thenear/far range border depth (e.g., 240 microns), the frequency of thetransducer (e.g., 20 MHz), the filter bands, and the types of tissuecharacterized, etc. However, in accordance with exemplary embodiments,the factors and corresponding criteria are designated to overcome atleast one of the two following false tissue characterization artifacts:(1) the false identification of necrotic core (NC) speckle at thelumen-tissue boundary in a vessel that is prone to acute rupture; and(2) excessive identification of NC speckle in dense calcium (DC). In theillustrative example, both types of false identification are reduced bythe combination of range/filter characteristics and generating first andsecond tissue/plaque classification criteria for near and far rangetissue/plaque classification. The differences between the twocharacterization criteria are generally universal and arise from boththe different ranges of echo signal information and the applied filterbands.

The near/far border for designating the end of a first range and thebeginning of a second range, 240 microns in the illustrative example,varies in alternative embodiments. The selection of a particular depthfor the first range is potentially influenced by the type of transducer,the imaging application, and the purpose for which the images are beinggenerated.

Furthermore, the band of the near range filter is modified in accordancewith alternative embodiments. For example, in an alternative embodimentthe near range filter band is 15-24 MHz. However, in each embodiment,the near range filter band is narrower than, and falls completelywithin, the far range filter band.

It is noted that the identification of distinct filters is intended toemphasize the presence of two distinct filter bands applied to differentranges of echo information. For example, in an exemplary embodiment twodistinct filters apply each of the two distinct filter bands. However,in an alternative embodiment, a single, multiple/variable band filterapplies differing filter bands to different ranges of echo signalinformation. Therefore, the near/far range filters are, in thealternative embodiment, a single filter that is configured in a firstinstance to apply a near range filter band to near range echoinformation, and configured in a second instance to apply a far rangefilter band to far range echo information.

Having described an exemplary system including near/far range filtersand associated criteria, attention is directed to FIG. 3 wherein a setof steps summarize an exemplary method for generating a blood vesselcross-section image that graphically depicts characterized tissue/plaqueusing the aforementioned dual filters and tissue classificationcriteria. Initially, during step 300, a set of IVUS echo information isobtained for a blood vessel cross section. Thereafter, at step 310 alumen-tissue border is identified for each radial direction around thetransducer probe (e.g. probe 122) that obtained the ultrasound echoinformation. Thereafter, during step 320 the first/near range filter isapplied to near range echo information provided by the IVUS console 110.The near range echo data corresponds to a portion of each echo signalsegment that begins at the lumen-tissue border and ends at the near/farrange border depth (e.g. 240 microns from the lumen-tissue border).Thereafter, during step 330 a near range tissue/plaque characterizationcriterion is applied to the filtered near range echo informationrendered during step 320. Thus, during step 330 a near rangetwo-dimensional tissue characterization image is created that begins atthe lumen-tissue border and extends to a depth of the near/far rangeborder (e.g., 240 microns). The near range image is stored in the datastorage 134.

During step 340 the second/far range filter is applied to the far rangeecho information provided by the IVUS console 110. In the illustrativeexample, the far range echo information is the remaining/distant portionof each echo signal segment that was not processed during steps 320 and330. Thereafter, during step 350 a far range tissue/plaquecharacterization criterion is applied to the filtered far range echoinformation rendered during step 340. Thus, during step 350, a far rangetwo-dimensional tissue characterization image is created that begins atthe near/far range border. The far range image is stored in the datastorage 134. Upon completion of step 350, the tissue/plaquecharacterization application has generated a complete image from twodistinctly processed near/far range echo information sets. It is notedthat while two filters and criteria are used in the illustrativeexample, in alternative embodiments three or more filters and/orcriteria are used to render a complete image of a region of interest.

Finally, during step 360, the complete image is converted/translatedinto appropriate image data for display on a graphical output device. Asthose skilled in the art will readily appreciate the form of the imagedata will vary in accordance with the form of storage file and intendeduse (e.g., tif, pdf, bmp, jpeg etc.). Thus, the manner in which the datastored will vary in accordance with various alternative embodiments.

It is noted that while the illustrative embodiment discloses the use oftwo ranges, filters and criteria, in alternative embodiments moreranges, filter bands, and/or associated criteria are used. It is alsonoted that while each range is assigned a dedicated filter band andcharacterization criterion, in alternative embodiments two ranges sharea single filter or alternatively a single characterizationequation—though such alternative embodiments will include at least twodistinct filter bands or two distinct characterization criteria that areapplied to at least two different ranges. For example, a single filterband renders data in two ranges that are processed according to twodistinct characterization criteria. Alternatively, two filter bands areapplied to near and far range data, and the two distinct filtered datasets are processed according to a single characterization criterion.

Furthermore, the illustrative embodiments are directed to characterizingvascular tissue/plaque. In alternative embodiments the disclosedmultiple filters/criteria arrangement are incorporated intocharacterization applications for characterizing: non-vascular canceroustissue (cancerous, benign), from the prostate, breast or other parts ofthe body. Also myocardial tissue is identifiable using theabove-described arrangement (e.g., healthy myocardial tissue, diseasedmyocardial tissue, ablated myocardial tissue, unablated myocardialtissue. Other vascular tissue is similarly characterized including:blood, thrombus, organized thrombus, unorganized thrombus, thrombusunder an intimal flap, fibrous cap of an occlusive thrombus,fibro-lipidic tissues, calcified necrotic tissues, collagen,cholesterol, compositional structures (lumen, vessel wall,medial-adventitial boundary, etc.). The system is also potentially usedto identify materials found in patients including, stent materials.

Systems and their associated components have been described herein abovewith reference to exemplary embodiments of the invention including theirstructures and techniques. It is noted that the present invention isimplemented in computer hardware, firmware, and software in the form ofcomputer-readable media including computer-executable instructions forcarrying out the described functionality. In view of the many possibleembodiments to which the principles of this invention may be applied, itshould be recognized that the embodiments described herein with respectto the drawing figures are meant to be illustrative only and should notbe taken as limiting the scope of invention. Therefore, the invention asdescribed herein contemplates all such embodiments as may come withinthe scope of the following claims and equivalents thereof.

1. A method of characterizing tissue, comprising: collecting ultrasoundecho information from a portion of an imaged body; applying a firstfilter, having a first filter band, to a near range portion of theultrasound echo information to render near range filtered echoinformation; applying a first characterization criterion to the nearrange filtered echo information to render a near range tissuecharacterization image; applying a second filter, having a second filterband that covers a frequency range of the first filter band, to a farrange portion of the ultrasound echo information to render far rangefiltered echo information; and applying a second characterizationcriterion to the far range filtered echo information to render a farrange tissue characterization image.
 2. The method of claim 1 whereinthe first and second characterization criteria include frequency andpower response characteristics of the near and far range filtered echoinformation.
 3. The method of claim 1 wherein the tissue is vasculartissue.
 4. The method of claim 3 wherein the tissue comprises plaque. 5.The method of claim 1 wherein the first filter band is about 12 to 26MHz.
 6. The method of claim 1 wherein the ultrasound echo information isrendered from a 20 MHz transducer.
 7. The method of claim 1 wherein theultrasound echo information is rendered from a 45 MHz transducer.
 8. Themethod of claim 1 further comprising combining the near range and thefar range tissue characterization image information to render a singledisplayable image.
 9. The method of claim 1 further comprising at leastone additional applying a filter and applying a characterizationcriterion to at least a third range of echo information to render atissue characterization image corresponding to the third range.
 10. Themethod of claim 1 wherein the first and second filters are implementedby a single filter capable of applying multiple/variable filter bands.11. A computer-readable medium including computer-executableinstructions facilitating characterizing tissue, the computer-executableinstructions facilitating performing a set of steps comprising:collecting ultrasound echo information from a portion of an imaged body;applying a first filter, having a first filter band, to a near rangeportion of the ultrasound echo information to render near range filteredecho information; applying a first characterization criterion to thenear range filtered echo information to render a near range tissuecharacterization image; applying a second filter, having a second filterband that covers a frequency range of the first filter band, to a farrange portion of the ultrasound echo information to render far rangefiltered echo information; and applying a second characterizationcriterion to the far range filtered echo information to render a farrange tissue characterization image.
 12. The computer-readable medium ofclaim 11 wherein the first and second characterization criteria includefrequency and power response characteristics of the near and far rangefiltered echo information.
 13. The computer-readable medium of claim 11wherein the tissue is vascular tissue.
 14. The computer-readable mediumof claim 13 wherein the tissue comprises plaque.
 15. Thecomputer-readable medium of claim 11 further comprising combining thenear range and the far range tissue characterization image informationto render a single displayable image.
 16. The computer-readable mediumof claim 11 wherein the first and second filters are implemented by asingle filter capable of applying multiple/variable filter bands.
 17. Asystem for generating visual representations of characterized tissuebased upon ultrasound echo information obtained from a portion of animaged body, comprising: a first filter, having a first filter band, forapplication to a near range portion of the ultrasound echo informationto render near range filtered echo information; a second filter, havinga second filter band that covers a frequency range of the first filterband, for application to a far range portion of the ultrasound echoinformation to render far range filtered echo information; and a set ofcharacterization criteria including: a first characterization criterionfor application to the near range filtered echo information to render anear range tissue characterization image, and a second characterizationcriterion for application to the far range filtered echo information torender a far range tissue characterization image.
 18. The system ofclaim 17 wherein the first and second characterization criteria includefrequency and power response characteristics of the near and far rangefiltered echo information.
 19. The system of claim 17 wherein the tissueis vascular tissue.
 20. The system of claim 17 wherein the first filterband is about 12 to 26 MHz. 21-26. (canceled)